1. Field of the Invention
The present invention relates generally to control valves and, more particularly, to such valves that are associated with a pressure reducing medium for minimizing damaging cavitation effects and for controlling line pressures.
2. Description of the Prior Art
A fluid flowing through a transmission system, such as a pipeline, possesses both potential energy (pressure) and kinetic energy (velocity). A rotary control valve installed in a pipeline can effectively control the pressure, volume, and/or the velocity of the fluid flowing through the pipeline. As the flow is throttled through the valve's restricted orifice, its velocity increases and reaches a maximum value at a point at which minimum pressure occurs.
But increasing flow velocities tends to create in the pipeline system excessive noise, flashing, cavitation, erosion, vibration, and/or flow instability. Thus, cavitation occurs when the pressures on the opposite sides of the valve's orifice are above the liquid's vapor pressure, but the minimum pressure drops to below the liquid's vapor pressure. Then bubbles form at the point of minimum pressure and move downstream to the point of pressure recovery where they implode. Under other pressure conditions, flashing occurs, and the generated bubbles do not collapse downstream.
Also, it is believed that sonic flow of gases and vapors creates high noise levels as a result of shock waves, which impinge on the slower moving fluid downstream of the control valve.
In U.S. Pat. No. 3,424,200 is disclosed a rotary disc valve having a tapered arcuate orifice which regulates flow according to the adjusted position of the disc. A plurality of small axial holes are provided around the edge of the orifice such that only a very small portion of the flow can take place through these holes. It is alleged that the very limited flow through these holes can avoid some of the undesirable cavitation effects due to the implosions of the bubbles downstream of the orifice disc.
In my U.S. Pat. No. 4,212,321 is described a spherical control valve which is rotatably mounted inside a spherical casing. The control element has an axis of rotation which is coaxial with the casing's main axis. Extending through the valve's pressure-reducing medium, along a diameter thereof, is a straight tube. A pair of elongated tapered orifice slots extend circumferentially in opposite directions from the opposite open ends of the tube. These slots form a straight flowway between the inlet and outlet flow ports. In the fully-open position, the tube becomes in alignment with the diametrically-opposed inlet and outlet ports. In its preferred embodiment, the entire wall of the tube is perforated and is surrounded by a perforated sleeve which acts as a pressure-reducing medium. But the tube's perforated wall becomes itself subjected to great stresses due to the large pressure drops thereacross. Also, because the tube's axis makes an acute angle with the axis of rotation, (1) the valve's casing must be made sufficiently large and heavy to provide the necessary reaction torques, and (2) undesirable turbulence and vibration occur.
In other well-known cylindrical and spherical control valves utilizing a pressure-reducing medium, fluid flows only through a small portion of the medium as the fluid barely starts to flow. For high velocity flow, the available pressure reducing medium is at a minimum, which creates excessive noise. In the fully-open position, the full bore flow becomes curved, which creates turbulence. In other such known valves, the full bore flow is through the entire pressure-reducing medium even when the flowway is fully open, resulting in destructive pressure drops, which were considered unavoidable.
For the above and other well-known reasons, known rotary control valves create a constant need to maintain a large inventory of spare parts to keep them operational, and sometimes to surround them with noise-suppressing structures, especially when they are operated close to inhabited areas, where strict environmental codes exist.
The spare part problem is very serious because in many parts of the world they are not available and must be flown in from far away places.
In sum, known control valves of the foregoing types are noisy, cumbersome, utilize excessive energy, are expensive to maintain, and require an excessive inventory of expensive replacement parts.
The objects of the present invention include the following: to provide a new and improved rotary control valve which allows through-conduit, unobstructed, undivided, straight flow when the valve control element is fully open; which allows parallel laminar flow through the entire body of a pressure-reducing medium for the entire flow regardless of volume; which features a substantially balanced, linear, dynamically-stable, and high-resolution flow over a wide throttling range up to 315.degree., and which can be constructed and operated at only a fraction of the corresponding costs for making and operating conventional such rotary control valves having comparable low noise level characteristics.